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  My A-7 bubble sextant has an astigmatizer that stretches out the image
of she sun horizontally so that it extends all the way across the bubble
and out on both sides. This helps you to judge that you have the sun
aligned with the center of the bubble. It can be used with stars but I
haven't tried it yet.

I could find no mention of astigmatizers in Bowditch or Dutton.

gl


On 9/22/2010 12:19 AM, George Huxtable wrote:
> The Admiralty Manual of Navigation, 1938, vol. 1, has an appendix on the
> sextant, containing these words, on page 390-
>
> "There are, on the market, various sextant attachments of which the
> following are examples-
> Power 6 prism star monocle
> Double star prism, for use with cloudy horizons.
> Astigmatiser, foe elongating the image of a star."
>
> I think that astigmatiser (or perhaps astigmatizer to Americans) is the
> right word to use, and Greg might find it to work better when searching
> than the stigmatizer that he referred to.
>
> I have an English Vernier sextant, around 90 years old, which has in the
> side of the box a slot containing a little "lens" that can fit over the
> eyepiece. At a first glance, this looks like simple plain glass, but it has
> a mark on the rim which is clearly there to show which way up it should go.
> I have always taken it to be a star astigmitiser, though have never
> bothered to try it out on a star. If it is, it has a very weak action,
> because it has no discernable effect on viewing of ordinary objects.
> Unfortunately, stars here are in short supply at present. Whether my old
> eyes are good enough to tell the difference is a question.
>
> I doubt whether spreading a star image into a short streak is intended to
> avoid the need for rocking the sextant; but on the other hand, I'm unsure
> what other benefit is intended.
>
> George.
>
> contact George Huxtable, at  george@hux.me.uk
> or at +44 1865 820222 (from UK, 01865 820222)
> or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
> ----- Original Message -----
> From: "Greg Rudzinski"
> To:
> Sent: Tuesday, September 21, 2010 8:42 PM
> Subject: [NavList] Re: Benefits of Stigmatizing
>
>
> Jeremy,
>
> I have never seen a double prism on a sextant so it is good to hear your
> first hand experience with this attachment. My original question though
> pertains to an optical lens that stretches out a point of light into a line
> which is perpendicular to the sextant frame. Modern sextants don't seem to
> have this stigmatizing lens and I was wondering if this was to lower cost
> or because there is no real benefit to the star observation. I too prefer
> rocking the sextant to consistently determining the vertical.
>
> Greg Rudzinski
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